The making of optical fiber cables thin in diameter, lightweight, and high in optical wiring density is being pursued to reduce cable prices and laying costs, and there have been stringent demands for making polyethylene (PE) spacers, which accommodate optical fibers, thin in diameter as well.
Meanwhile, with recent aerial optical fiber cables, intermediate post-branching performance of the optical fibers is being required in addition to high optical wiring densities, and this has lead to the frequent use of SZ-type optical fiber cables, which use a PE spacer (SZ spacer), having grooves that accommodate the optical fibers and are alternately inverted in spiraling direction in SZ-like manner, and with which a plurality of tape-form optical fibers are accommodated within each groove of the spacer.
In the case where a rigid optical tape is to be accommodated used in an SZ spacer, the dimensions of an accommodating groove must enable the securing of adequate space for allowing the twisting of the tape. Also, though the polyethylene resin that comprises the rib undergoes three-dimensional molding shrinkage (sum of the shrinkage due to recrystallization during solidification and volume shrinkage due to lowering of the resin temperature) in the process of extrusion molding, unlike in the case of a unidirectionally stranded spacer, with which there is no allowance for shrinkage of the ribs in the length direction, in the case of an SZ spacer, lengthwise shrinkage of the ribs is possible in the form of short-cutting the inversion curve at just the inversion part, and as a result, the ribs can collapse towards the inner side of the inversion curve.
This phenomenon becomes more prominent when the ribs are made thin in root thickness and, along with the abovementioned securing of groove space, this has been a factor that has hampered the making of SZ slots thin in diameter.
It is considered that besides the molding shrinkage of the resin, the collapsing of the ribs maybe caused by the mutual pulling of the coating resins, due to differences in the drawdown of the resin, etc., in the process of performing extrusion coating from a nozzle.
In the case of an optical fiber cable that uses a thin-diameter SZ spacer, with which the minimum rib thickness at-the root, etc. of the rib is thin, the inversion pitch must be made short in order to allow for extra lengths of optical fiber, and since the inclination angle of the groove at the inversion part thus becomes large, the transmission loss is increased inevitably.
An object of this invention is to provide an SZ-spiral-grooved spacer for optical fiber cable, with which the groove inclination at the inversion parts of the SZ spacer is restricted and is low in the increase of transmission loss, and an optical fiber cable that uses this spacer to realize the making of optical fiber cables thin in diameter.